Work on the vdec stalled due to an upstream compliance review of the kernel APIs involved and since rework would be required it was better to wait until it completed before continuing. There are also issues in the firmwares - it's the reason why HEVC is (still) missing. The API reviews are now done and updates for MPEG2 and H264 codec have already been submitted upstream, but those codecs are the easy ones and VP9/HEVC are a lot more complicated. Some work on retooling ffmpeg and Kodi around the compliance changes has already started.
NB: Team Kodi will bring forward the v19 release to get the hugely disruptive Python3 change into public view and the ETA on first alpha is within the next month as Python2 will be EOL on 1st Jan. The GBM/V4L2 work we are championing on ARM platforms will not align with the reschedule so v19 will be an alpha/beta release for ARM devices and v20 becomes the target, on similar dates to the original v19 schedule.